Joints are the points where two or more bones meet, facilitating movement and maintaining posture. They are classified in several ways, including by the degree of mobility they permit. Synovial joints are the most mobile, functionally classified as diarthroses, meaning they are freely moveable. These joints are further categorized based on the number of axes, or planes, around which they can rotate.
Structural Characteristics of Biaxial Joints
A biaxial joint is a type of synovial articulation that allows movement around two separate, perpendicular axes or planes. This structural arrangement provides more mobility than a simple hinge joint but less than a ball-and-socket joint. The shape of the articulating bone surfaces restricts motion to these two main directions.
Typically, one bone has an oval or concave surface that fits into a complementary convex or elliptical depression on the adjacent bone. Ligaments surrounding the joint guide and limit the range of motion, confining movement to these two primary planes. This design prevents the joint from rotating on a third, or longitudinal, axis.
The Two Axes of Movement
The two axes of rotation permit two distinct pairs of movements, occurring in relation to the body’s standard anatomical planes. The first is the mediolateral axis, oriented side-to-side, which permits movement within the sagittal plane. Motion around this axis involves flexion, which decreases the angle between bones, and extension, which increases the angle.
The second axis is the anteroposterior axis, running from front to back, which allows for movement within the frontal or coronal plane. Movement along this axis involves abduction (moving away from the midline) and adduction (moving toward the midline). The sequential combination of all four movements allows a biaxial joint to perform circumduction, where the distal end of the limb moves in a circle.
Subtypes and Key Locations
Biaxial joints are represented by two primary structural subtypes: the condyloid joint and the saddle joint. The condyloid joint, sometimes called an ellipsoidal joint, features an oval-shaped condyle of one bone resting within an elliptical cavity of another. This structure allows for the two axes of movement but inhibits rotation.
A prominent example of a condyloid joint is the radiocarpal joint, connecting the radius bone with the carpal bones of the wrist. The metacarpophalangeal joints, commonly known as the knuckles, are also classic examples.
The saddle joint, or sellar joint, is structurally unique because the articulating surfaces are reciprocally shaped. Each bone surface is concave in one direction and convex in the other, resembling a rider in a saddle. This interlocking structure provides a slightly greater range of motion than the condyloid joint. The most well-known example is the first carpometacarpal joint at the base of the thumb.
Contextualizing Joint Mobility
The classification of biaxial joints is best understood when compared to other synovial joints based on their mobility. Joints that permit movement around only one axis are uniaxial joints. The elbow joint, which functions as a hinge joint allowing only for flexion and extension, is a common example.
In contrast, joints that permit movement around three axes are known as multiaxial joints. These joints offer the greatest range of movement, enabling motion in the sagittal, frontal, and transverse planes. Ball-and-socket joints, such as the hip and shoulder, are primary examples, permitting a full spectrum of movements, including rotation. Biaxial joints occupy a functional middle ground, providing maneuverability in two dimensions without the rotational capacity of a multiaxial joint.